When you create order lines, the delivery date is transferred from the order header to the lines. You can automatically update the delivery date in the order lines with that in the order header by setting the parameter in the Update order linesform, accessed through the Updatestab in the Accounts receivable parametersform.

Note
It is important that you specify the correct delivery date for the lines because the order lines are used in requirement calculations, with the expected inventory issue of the item on the specified date.